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A four-quadrant gate is a type of boom barrier gate protecting a grade crossing. It has a gate mechanism on both sides of the tracks for both directions of automotive traffic. The exit gates blocking the road leading away from the tracks in this application are equipped with a delay, and begin their descent to their horizontal position several seconds after the entrance gates do, so as to avoid trapping highway vehicles on the crossing. Many people consider four-quadrant gates to be safer than two-quadrant gates because they prevent drivers from illegally driving their vehicles around lowered gates to try to beat a train.
In the UK, such crossings are categorised as 'Manually Controlled Barriers' (MCB) because they are always manually controlled, usually from a signal box. Some are known as MCB-CCTV level crossings, because they are supervised by video link to the signal box from which they are remotely controlled.
The first four-quadrant gate in the United States was installed in 1952. They have become common for new installations and replacements since. Unlike the UK, many are automated by the rail line's signal system. The first quad gate installation in the country with sensors to detect vehicles stopped in the tracks was installed in Groton, Connecticut in 1998. Eight of the eleven remaining grade crossings on the Northeast Corridor now have such setups.